In the motion picture industry, it is common practice to make prints of a duplicate motion picture negative. This is done for several reasons, the most significant being that the original negative is preserved as a master record. Thus, typically, the original negative is copied onto an intermediate negative film one or more times. That copy, in turn, is used to make positive prints which are projected in a movie theatre.
Copying of the original negative record to make an intermediate negative, or printing of an intermediate negative record to give a positive is accomplished by illuminating the record to be copied with a lamp so that radiation passing thru the record to be copied impinges on the light sensitive material in which the reproduction will be formed. This can be done at multiple locations.
It is common practice to employ as the source of illumination three separate portions of the visible spectrum; one which has its peak in the blue region of the spectrum, between about 400 and 500 nm, one which has its peak in the green region of the visible spectrum between about 500 and 600 nm, and one which has its peak in the red region of the visible spectrum between about 600 and 700 nm. The exposing radiation could be provided by three separate light sources of the appropriate spectral distribution, or it could be provided by a single white light source which is split into three separate beams which are filtered with red, green or blue filters before they illuminate the record being copied. Typically each of the exposure sources, the dyes which provide the absorption in the record to be copied and the sensitivity of the light sensitive element onto which the copy is made are chosen so that their peaks approximately match.
The type of exposure source described above is referred to in the art as an additive lamp house. It is the one which is commonly used in the motion picture industry for reproduction of negatives and preparation of prints. There are subtle variations in the output of individual additive light sources. Thus, when an original record to be copied is exposed to one additive light source, it may modulate a somewhat different spectral wavelength distribution than it does when exposed to a different additive light source. As a result, the spectral wavelength distribution of the light reaching the light sensitive material and, consequently, the color reproduction in the copy can be different depending on the particular printer used. This can be a problem, especially when multiple printing stages occur.
Our parent application Ser. No. 718,685, cited above describes a system using selective filtration with narrow band filters to modify the light modulated by the record being copied to provide a reproduction using an subtractive light source that is compatible with a reproduction made using an additive light source.
We have now found that when these selective filters are applied to different additive light sources the range of variation among them is reduced, resulting more consistent reproductions when using different additive light sources.
While narrow band filters have been used previously to modify reproductions, they have been used in different ways and for different purposes than they are used in accordance with the present invention.
Hehn U.S. Pat. No. 3,085,468, issued Apr. 16, 1963 describes the use of dichroic filters in printing photographic originals in place of the red, green and blue filters commonly used in additive printing systems.
Krause U.S. Pat. No. 4,359,280, issued Nov. 16, 1982 describes the use of dichroic filters, and other narrow band filters to modify contrast by shifting peak intensity characteristics of the light modulated by the record being copied. This is different from the present invention in which the peak intensities are not changed.